call magit-OPNAME-action-hook, fixes #802

Since 46467190 `magit-section-action' was looking for a section of
type (run-hook-with-args-until-success magit-*opname*-action-hook)
instead of actually calling the hook.
make :eval kludge added in last commit unnecessary

Instead of adding `(:eval ...)' as another special kind of
`magit-section-case' CLAUSE, run the `magit-section-action'
hook inside the existing `t' special clause.
magit-git{,k}-executable: store full path, not just name

This causes the full path to be displayed in *magit-process*, which is
a good thing, as it will help debugging windows problems.  If someone
complains about that we can still add an option to only display the
basename.
add magit-quote-curly-braces

This fixes issue #816 while keeping #757 in mind: on Windows curly
braces sometimes have to be quoted sometimes not.
magit-run-gitk: on Windows: fix putting sh on path

Also cleanup a bit.
Merge branch 'fix-windows'
make it official that only Git >= 1.7.2.5 is supported

See #709 for discussion.
recommend git-1.8.2

It contains a remote helper for bzr repositories which benefits any
git-loving user of Emacs-bzr.